The National Highway Traffic Safety Administration wants Chrysler to answer questions by July 16 about why it is taking so long to produce the trailer hitches necessary to protect fuel tanks on 1.56 million old Jeep SUVs.

More than a year after agreeing to install trailer hitches the automaker has not begun the repairs on 2002 through ’07 Jeep Libertys and 1993 through ’98 Grand Cherokees.

“Although NHTSA acknowledges that recall campaigns may have low completion rates for any number of reasons, the agency has no intention of allowing Chrysler, or any other manufacturer, to delay recall completion to the detriment of safety,” Chief Counsel Kevin Vincent said today in a letter to Chrysler.

The 12-page letter reveals the administration’s frustration with Chrysler, which refused to conduct crash tests with the trailer hitches. The friction between the company and the administration has boiled for more than a year because Fiat and Chrysler CEO Sergio Marchionne argued that the vehicles are not defective.

The Jeeps have rear-mounted fuel tanks that the traffic safety administration determined put the vehicles at a higher risk than other SUVs for rear-end explosions. The fuel tanks were redesigned and placed in new positions in more recent models of the Grand Cherokee and the Liberty, which Chrysler stopped making in 2013.

Chrysler refused. So the administration conducted its own crash tests last August and gave the automaker permissionto move forward with the trailer hitch repairsin January.

According to NHTSA, Chrysler didn’t select a supplier until December and issued its first purchase order in January. The agency said Chrysler’s supplier did not begin making the trailer hitches until May — nearly 12 months after the automaker agreed to conduct the safety campaign.

At its current pace, NHTSA, estimates it will take nearly five years for Chrysler to make the required quantity of hitches for the Grand Cherokee and two years for the Liberty.

Now, Chrysler has until July 16 to provide documents about how it selected the suppliers and financial projections about the cost of the program.

NHTSA also wants to know why Chrysler hired only one supplier to produce the trailer hitches. Chrysler told the Free Press it hired several suppliers and said the repair program will begin in August.

Last August, the safety administration conducted eight crash tests and concluded that trailer hitches will reduce the risk of fuel tank ruptures and fires in low- to medium-speed rear-impact crashes. The hitch absorbs much of the impact and prevents another vehicle from sliding under the Jeep SUVs and hitting the fuel tank.

Chrysler said it cooperated with the traffic safety administration during the investigation and “has been working with NHTSA all along in this process.”

In fact, Chrysler said the crash test data with trailer hitches, “Reflects Chrysler Group’s longstanding position — supported in the public record by real-world data — that the vehicles are not defective. They are among the safest in their peer groups and met or exceeded the standards in effect at the time they were first sold.”

Chrysler also said it selected suppliers and began production as fast as it could.

“Launching a safety recall demands complex engineering and close coordination with NHTSA well before an auto maker accumulates replacement parts,” Chrysler said in its statement. “To accommodate the high-volume production required for this campaign, (we) had to find multiple new supplier partners to (make) this part that far exceeded normal demand.”